In general, TFT liquid crystal panels are constructed by filling liquid crystals between an array side substrate having TFT devices built therein and a color filter substrate. They are based on the active matrix addressing scheme where voltage is applied by TFTs for controlling the alignment of liquid crystals.
In the manufacture of the array side substrate, patterns are formed in plural layers on a mother glass such as non-alkaline glass by repeating light exposure through originals having circuit patterns drawn thereon, known as large-size photomasks. On the other hand, the color filter side substrate is manufactured by a lithographic process known as dye immersion process. In the manufacture of both array and color filter side substrates, large-size photomasks are necessary. For performing light exposure at a high accuracy, such large-size photomasks are typically made of synthetic quartz glass characterized by a low coefficient of linear thermal expansion.
So far, liquid crystal panels have progressed to higher definitions from VGA to SVGA, XGA, SXGA, UXGA and QXGA. It is believed that degrees of definition ranging from 100 pixels per inch (ppi) class to 200 ppi class are necessary. Accordingly, a strict exposure accuracy, especially overlay accuracy, is imposed on the TFT array side.
Some panels are manufactured using the technology known as low-temperature polysilicon. In this case, it has been studied to bake a driver circuit or the like on a peripheral portion of glass, aside from the panel pixels, which requires light exposure of higher definition.
For large-size photomask-forming substrates, it is known that their shape has an influence on the accuracy of light exposure. As shown in FIG. 1, for example, when light exposure is performed using two large-size photomask-forming substrates having different flatness, the patterns are shifted due to the difference between light paths. More specifically in FIGS. 1A and 1B, broken lines represent light paths when light advances straight and the mask is ideally planar. Actually, light paths are shifted outward or inward, as shown by solid lines, depending on whether the substrate upper surface is concave or convex. Also, for an exposure apparatus using a focusing optical system, there arises a phenomenon that the focus is shifted from the exposure plane, resulting in degraded resolution. Thus, for light exposure of higher accuracy, there is a need for large-size photomask-forming substrates having a higher flatness.
To implement the multiple pattern technology through a single light exposure for increasing the productivity of panels, there arises a demand for a large-size photomask-forming substrate having a diagonal length as large as 1500 mm. Both a larger size and a higher flatness are required at the same time.
Large-size photomask-forming substrates are generally manufactured by lapping plate-shaped synthetic quartz with a slurry of loose abrasives (e.g., alumina) suspended in water for thereby removing irregularities on the surface, then polishing with a slurry of abrasives (e.g., ceria) suspended in water. To this end, a double- or single-side processing machine is used.
However, these processing methods, which utilize for flatness correction the reaction force against the elastic deformation generated when the substrate itself is forced against the processing platen, have a drawback that as the substrate size becomes larger, the reaction force considerably decreases, leading to a reduction of the ability to remove moderate irregularities on the substrate surface. FIG. 2A illustrates the shape of a substrate 1 when held vertically. FIG. 2B illustrates the shape of the substrate 1 during processing, indicating that the substrate 1 conforms to the platens. FIG. 2C illustrates the reaction force against the elastic deformation of the substrate 1 at that time, indicating more processing by this force (ΔP) than other positions.
It is also a common practice to improve flatness using a surface grinding machine. In general, the surface grinding machine is adapted for a workpiece to traverse a predetermined gap between a workpiece-mount table and a grinding tool, for removing those portions of the workpiece which are greater than the predetermined gap. If the workpiece on the rear surface is not provided with a sufficient flatness, no improvement in flatness is achievable. This is because the workpiece is urged against the workpiece-mount table due to the grinding force of the grinding tool, and as a result, the flatness of the front surface conforms to the flatness of the rear surface.
To solve these problems, JP-A 2003-292346 corresponding to US-2003-0143403-A1 and EP 1,333,313 A1 proposes a method of processing a large-size photomask-forming substrate by partially removing raised portions and thick portions on the substrate by means of a partial processing tool. When grinding or sand blasting is utilized as the partial processing tool, however, the partial processing may cause brittle fracture to the substrate, whereby there is a possibility of generating microcrack-like defects on the substrate surface. When it is desired to produce a defect-free large-size substrate, such crack-like defects must be removed by polishing by means of a double- or single-side polishing machine following the partial processing. The polishing machine used following the partial processing needs a more quantity of labor and time for the management and maintenance of the accuracy of the polishing machine so that the polishing may not exacerbate the flatness of the substrate and/or the accuracy of thickness variation. If the flatness of the substrate or the accuracy of thickness variation is exacerbated and shifted from the desired value by the polishing following the partial processing such as sand blasting, then it becomes necessary to carry out again partial processing such as sand blasting and subsequent polishing. It would be desirable to have a processing method capable of tailoring accuracy without brittle fracture and without a need for subsequent polishing.
Also proposed is a processing tool having an abrasive cloth attached to a platen so as to avoid any brittle fracture. Since the processing speed is gradually reduced due to the wear of the abrasive cloth during the process, the processing tool must be replaced frequently, which requires a labor and a time. There is a desire to have a processing method capable of partial processing at a constant processing speed with an economic advantage without brittle fracture and without a need for subsequent polishing.